1. Field of the Invention
The present invention relates to a pattern inspecting apparatus for inspecting a pattern defect of a sample on which a fine pattern is formed such as a photo mask for use in a process for manufacturing a semiconductor integrated circuit.
2. Description of the Related Art
In a process for manufacturing a large scale integrated circuit (LSI), an optical reduction exposure device (stepper) for circuit pattern transfer uses a photo mask having a circuit pattern magnified to 4 to 5 times as an original block. A request for integrity (pattern precision and no defect, etc.) for this photo mask is very higher and higher year by year. In recent years, a pattern transfer is carried out in the vicinity of a critical resolution of the stepper with super-fining and high integration, and a photo mask with high precision is used as a key of manufacturing a device. Thus, development of devices for inspecting such a defect is actively underway, and such devices are practically used.
Among them, the improvement of performance of a mask defect inspecting apparatus for detecting a defect of a super-fine pattern is essential for the improvement of the yield of short-term development and manufacture of an advanced semiconductor device.
As a conventional mask defect inspecting method, there is provided a method (a die-to-die comparison scheme) for scanning and observing two chips on which the same pattern has been depicted by a sensor having a plurality of pixels, respectively, and comparatively detecting a difference between both of these chips in accordance with a proper defect detecting algorithm (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 10-282008).
In order to efficiently fetch a chip image, it is preferred to penetrate and scan a plurality of dies which continuously exist in a stripe lengthwise direction at one time, and acquire an image. Because of this, there is employed a method for continuously photographing images, fetching the photographed images in a memory, and comparing images on the memory with each other at the same time as the fetching or after completing fetching on a one stripe by one stripe basis.
The images fetched by a sensor is provided as two-dimensional multi-gradation digital images when one pixel of the sensor is defined as a unit (mesh). On the other hand, a dimension of each die (chip) or gap dimension between the dies and an image pixel dimension do not have a relationship of integer multiples. Thus, if a first die image and a second die image are superimposed on each other on a pixel by pixel basis, each image pixel appears as if a pattern displacement has occurred.
Conventionally, in order to practically carry out die comparison, one die image is corrected so as to eliminate this pattern displacement. This method is provided for cutting a mesh again on a principle of weighting one die image by a plurality of image pixels, and obtaining a weight average or the like, and for comparison of the pixels of die images. A defect determination algorithm for correcting the positions of pixels of nominal die images, and then, making die-to-die comparison, has been applied.
In cutting the mesh again, interpolation of pixels of die images is carried out, and then, a type of smoothing processing is applied. Therefore, there has been a side effect that an image whose pattern displacement has been corrected blurs (is averaged) principally, an essential defect signal for quantization error, in particular, a signal intensity of a fine defect is dulled, or a profile of a pattern edge is changed, and there has been a limitation to the improvement of inspection sensitivity.
Further, even when dies exist repeatedly in an inspection stripe widthwise direction (in a Y direction), a dimensional relationship between a mesh and a die of an image pixel does not always have a relationship of integer multiples, and thus, there is a problem similar to an X direction. Therefore, the fact that an image produced in a process for cutting the mesh in the Y direction again has been a limitation to the improvement of inspection sensitivity.